How to Manually Add Store Credit Option to the Magento Checkout

Magecredit should automatically integrate into your shopping cart.
In the event that it doesn't, please see our
Display Troubleshooting page.

In general, Magecredit requires the store credit field to represent "use" or "don't use".
This is done basically by just setting the ['payment']['use_customer_balance'] request value
to 1 if you want to use store credit, or 0 to not use store credit after a quote/order total
update - just like Magento's core checkout does it. Assuming the 3rd party checkout module
does not hinder any observers or checkout blocks, Magecredit will observe the quote
and update the total.
You can look at app\design\frontend\base\default\template\customerbalance\checkout\onepage\payment\additional.phtml for reference.